1. Field of the Invention
This invention relates to the annealing of compound semiconductor materials and more specifically to the laser annealing of such materials using short term energy deposition.
2. Description of the Prior Art
Semiconductor devices generally require materials which are doped with appropriate elements in order to yield useful electrical properties. Numerous techniques are available for the fabrication of such doped semiconductor materials. Among such techniques are liquid and vapor phase epitaxial growth, diffusion, and ion implantation. The ion implantation technique has specific advantages in that the dopant levels may be more reproducibly controlled with regard to concentration, purity and physical location. However, it is found that the implantation process seriously damages the crystal structure of the semiconductor material, resulting in significant deterioration of its electrical properties. Currently such damage is "annealed" by heating the implanted material, typically to values in excess of one half of its melting point temperature, for periods ranging from minutes to hours. Such annealing processes improve the electrical properties of the material. It is believed that this improvement may be associated with a readjustment of the semiconductor to a crystal state. However, useful electrical properties may be attained even though the final crystal state is not perfect.
Compound materials, such as gallium arsenide, present a specific problem when annealed using the prior art thermal processes. The relative ineffectiveness of such annealing processes, when applied to compound semiconductors, is believed to be associated with the difference in volatility between the various constituent elements of the compound semiconductor. For example, in a gallium arsenide substrate significant evaporation of arsenic, and even out-diffusion of some gallium, may occur during the heating required to anneal.
A recent development involves annealing of damaged crystals using short laser pulses (see, for example, Soviet Physics Semiconductor, Vol. 10, No. 3, March 1976, page 265 and references cited therein). Most of this work has been applied to the annealing of silicon. Additional work recorded in the Russian literature (Soviet Physics Semiconductor, Vol. 9, No. 7, page 946) indicates that the annealing process is effective when applied to compound semiconductors, such as gallium arsenide, as well. However, even annealing with short laser pulses was believed to result in significant loss of the constituent elements of the compound semiconductor, unless steps were taken to prevent such evaporative loss. This is demonstrated by the fact that the reported gallium arsenide experiments involve the utilization of a silicon nitride cap over the gallium arsenide surface in order to prevent evaporation during the annealing.